Use kth_distance in query

[azizkayumov]

This PR modifies query to use current distance to kth neighbor to early prune unnecessary node visits.

[codecov]

Codecov Report

Merging #62 (13dd4b6) into main (dad5656) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   77.91%   77.81%   -0.10%     
==========================================
  Files           3        3              
  Lines         335      338       +3     
==========================================
+ Hits          261      263       +2     
- Misses         74       75       +1     

Impacted Files	Coverage Δ
src/ball_tree.rs	77.68% <100.00%> (-0.15%)	⬇️

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[azizkayumov]

@msk is it related to this PR? From the benchmarks, Ball-Tree (in its current form) is always worse than the brute force (even in low dimensions), having kth_distance checked, Ball-Tree performs better in lower dimensions.

[azizkayumov]

Closing as obsolete, opened an issue for future readers #63.